1. Field of the Invention
The present invention relates to an axial turbine.
2. Description of Related Art
Gas turbines and steam turbines used in, for example, power plants are broadly classified into the following three types: an axial turbine, a diagonal turbine, and a radial turbine according to the direction in which a working fluid flows. The working fluid flows: in a direction along a rotational axis of a turbine in the axial turbine; in a direction diagonally expanding from the rotational axis of the turbine in the diagonal turbine; and in a radial direction with respect to the rotational axis of the turbine in the radial turbine. Of these three types of turbines, the axial turbine is particularly suitable for power plants having medium to large capacities and is widely applied to steam turbines in large-scale thermal power stations.
Recent growing interest in improved economy and reduced environment load has prompted a need for even greater power generation efficiency in power plants. Providing turbines with enhanced functions is thus an important issue to be addressed. Factors that govern turbine performance include stage loss, exhaust loss, and mechanical loss. It is considered to be effective to have a greater annulus area, specifically, to increase a blade height or a pitch circle diameter to thereby increase kinetic energy of the working fluid recovered in a last stage and to thus reduce the exhaust loss.
Increasing the annulus area, however, involves the following problems: (1) increased stress acting on a bucket and a rotor; (2) a stronger likelihood that loss will increase because of an inlet velocity becoming supersonic on a bucket tip side; (3) a stronger likelihood that separation will occur at an enlarged passage portion on an outer peripheral side; and (4) a stronger likelihood that an erosion amount will increase caused by droplets on the bucket tip side. Of the foregoing four problems, the problem of (3) “a stronger likelihood that separation will occur at an enlarged passage portion on an outer peripheral side” is extremely critical, because the problem not only constitutes a factor for occurrence of loss, but also can affect a flow pattern in a turbine stage disposed downstream of the separation.
As a solution, to the foregoing problem, a technique has been developed in which an annular baffle plate that follows a profile of a diaphragm outer ring is disposed between the diaphragm outer ring and a diaphragm inner ring in the turbine stage of the last stage (see, for example, JP-2013-148059-A). The technique causes the annular baffle plate to form an annular passage, thereby allegedly preventing occurrence of a separation and a reverse flow at an enlarged passage portion on an outer peripheral side.